This invention is concerned with biofiltration via a medium, usually a natural medium, which is effective to remove noxious odors and components from gases given off by a sewage treatment facility or industrial facility. Also, the invention concerns a composting arrangement. More specifically the invention concerns a modular support system for erecting a biofiltration or compost support bed in any desired size and shape, and with provision for control of gas distribution through the medium.
The general subject matter of this invention is addressed in U.S. Pat. No. 6,255,102, owned by the assignee of the present invention. That patent is incorporated herein by reference. This invention improves over the modular assembly shown in U.S. Pat. No. 6,255,102 by including an efficient structure and method for controlling the distribution of gases introduced into the plenum of space below the support system, so that gases are not unevenly and inefficiently distributed as they pass through the filtering medium.
Biological air pollution control has been used on an increasing basis in recent years to cleanse noxious and odorous gases, such as those emanating from a sewage treatment plant. The biological filtration systems or biofilters utilize a bed of stacked biofilter material such as compost, tree bark, peat, heather or soil, generally about three feet or more deep. The containment gas from the sewage treatment plant or other sources is blown through the biofilter material in an upward direction. The material in the biofiltration bed provides an environment for a diverse culture of microorganisms, which degrade the gaseous pollutants as they pass through the biofilter.
Such biofiltration or biological treatment of the gases is an inexpensive treatment method, but the raw gas stream must generally meet certain conditions: The pollutants should be water soluble and biodegradable, and free of toxic components. Also, there must be sufficient concentration of oxygen. Further, the gas stream should be nearly saturated with water (relative humidity at least about 95%), and at temperature between about 40° F. and 140° F. (more preferably between 50° F. and 100° F.). The odorous gas must also contain no more than small amounts of dust and grease.
These requirements usually dictate that the raw gas be preconditioned before entering into the biofilter bed. Preconditioning includes humidification, temperature control and removal of particulates.
In the biofilter itself, a biofilm covers the substrate of tree bark, peat, heather, etc. Water soluble air pollutants are absorbed into the biofilm and are decomposed into carbon dioxide and water by the microbes. The filter beds are often built on a single level, but where area is limited, multiple level biofilters have been constructed, with the raw gas stream divided and fed in parallel to the various levels of the biofilter system.
Regarding sizing of the biofilter beds, the height of the filter bed, i.e. the stack of organic material as the medium, is generally in the range of about 1 feet to 5 feet. Areas are up to tens of thousands of square feet. One rule of thumb regarding the calculation of needed area for a given waste air flow is that the “area load” or ratio between the waste gas flow rate and the filter area should be in the range of about 2.5 to 3.0 CFM/SF. The flow distribution and humidity of the inlet gas have to be carefully controlled, otherwise the filter bed will eventually clog and cease to function. Distribution throughout the area of the filter bed should be relatively even.
In addition to sewage treatment plants, biofilters are useful for treating the effluent gases from many other processes. These include factory farming, rendering plants, coffee roasters, foundries, composting plants, kraft paper drying and paint shops. The odorous emissions treatable include ammonia, hydrogen sulfide, amines, aldehydes, mercaptains, alcohols, disulfides, esters, fatty acids, unsaturated hydrocarbons, ketones, hexane, dichloromethane, formaldehyde, phenol, organic acids, acetone, toluene and others.
These biofilter systems are being used increasingly in treating the gases from sewage treatment processes and also some of the other processes listed above. Also, composting of natural materials can be conducted on a similar support bed, for active aeration of the compost material. There is a need for a relatively simple, inexpensive and modular approach for constructing a filter bed support for such processes.
Hallsten U.S. Pat. No. 5,454,195 discloses a modular containment system for preventing hazardous materials from leaching into the ground or otherwise escaping a defined containment area. That system includes peripheral modules and a liner to define the containment area, the modules being fillable by water or granular material, and the system has some relevance to the present invention described below.
Hallsten U.S. Pat. No. 6,255,102, referenced above, discloses efficient assemblies for erecting a biofiltration medium support, in several embodiments. The patent discloses a means and method for directing the flow of gases within the plenum comprising vanes which could be adjusted as desired, on erection of the filtration support, to deflect the moving gases. However, there is a need for a system capable of adjusting for proper air distribution after erection of the modular support structure, so that gas flow distribution can be tuned to maximize even distribution by actual testing after the structure has been fully erected. This is an object of the present invention described below.